This is one of a fairly large group of irregular -ir verbs that are all conjugated the same way. Other members of this group include sortir and servir. The most significant difference between these verbs' conjugation and that of the regular -ir verbs is that these verbs' conjugation does not use the infix -iss-. Further, this conjugation has the forms (je, tu) dors and (il) dort in the present indicative and imperative, whereas a regular -ir verb would have *dormis and *dormit (as in the past historic).